1. Field of the Invention
The present invention relates to an illumination system for a lithography apparatus, such as may be used to produce an illumination beam of radiation and in which the intensity distribution of a beam of radiation at a plane is controlled. More particularly, the invention relates to the use of the illumination system in a lithographic projection apparatus comprising:
a radiation system comprising an illumination system, for supplying a projection beam of radiation;
patterning means, for patterning the projection beam according to a desired pattern;
a substrate table for holding a substrate; and
a projection system for imaging the patterned beam onto a target portion of the substrate.
2. Description of the Related Art
The term xe2x80x9cpatterning meansxe2x80x9d should be broadly interpreted as referring to means that can be used to endow an incoming radiation beam with a patterned cross-section, corresponding to a pattern that is to be created in a target portion of the substrate; the term xe2x80x9clight valvexe2x80x9d has also been used in this context. Generally, the said pattern will correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit or other device (see below). Examples of such patterning means include:
A mask table for holding a mask. The concept of a mask is well known in lithography, and its includes mask types such as binary, alternating phase-shift, and attenuated phase-shift, as well as various hybrid mask types. Placement of such a mask in the radiation beam causes selective transmission (in the case of a transmissive mask) or reflection (in the cause of a reflective mask) of the radiation impinging on the mask, according to the pattern on the mask. The mask table ensures that the mask can be held at a desired position in the incoming radiation beam, and that it can be moved relative to the beam if so desired.
A programmable mirror array. An example of such a device is a matrix-addressable surface having a viscoelastic control layer and a reflective surface. The basic principle behind such an apparatus is that (for example) addressed areas of the reflective surface reflect incident light as diffracted light, whereas unaddressed areas reflect incident light as undiffracted light. Using an appropriate filter, the said undiffracted light can be filtered out of the reflected beam, leaving only the diffracted light behind; in this manner, the beam becomes patterned according to the addressing pattern of the matrix-addressable surface. The required matrix addressing can be performed using suitable electronic means. More information on such mirror arrays can be gleaned, for example, from U.S. Pat. Nos. 5,296,891 and 5,523,193, which are incorporated herein by reference.
A programmable LCD array. An example of such a construction is given in U.S. Pat. No. 5,229,872, which is incorporated herein by reference.
For the sake of simplicity, the rest of this text may, at certain locations, specifically direct itself to examples involving a mask table and mask; however, the general principles discussed in such instances should be seen in the broader context of the patterning means as hereabove set forth.
For the sake of simplicity, the projection system may hereinafter be referred to as the xe2x80x9clensxe2x80x9d; however, this term should be broadly interpreted as encompassing various types of projection system, including refractive optics, reflective optics, and catadioptric systems, for example. The radiation system may also include components operating according to any of these design types for directing, shaping or controlling the projection beam of radiation, and such components may also be referred to below, collectively or singularly, as a xe2x80x9clensxe2x80x9d. Further, the lithographic apparatus may be of a type having two or more substrate tables and/or two or more mask tables. In such xe2x80x9cmultiple stagexe2x80x9d devices the additional tables may be used in parallel, or preparatory steps may be carried out on one or more tables while one or more other tables are being used for exposures. Twin stage lithographic apparatus are described in for example, U.S. Pat. No. 5,969,441 and U.S. Ser. No. 09/180,011, filed Feb. 27, 1998, incorporated herein by reference.
Lithographic projection apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, the patterning means may generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (comprising one or more dies) on a substrate (silicon wafer) which has been coated with a layer of photosensitive material (resist). In general, a single wafer will contain a whole network of adjacent target portions which are successively irradiated via the projection system, one at a time. In current apparatus, employing patterning by a mask on a mask table, a distinction can be made between two different types of machine. In one type of lithographic projection apparatus, each target portion is irradiated by exposing the entire mask pattern onto the target portion at once; such an apparatus is commonly referred to as a wafer stepper. In an alternative apparatusxe2x80x94which is commonly referred to as a step-and-scan apparatusxe2x80x94each target portion is irradiated by progressively scanning the mask pattern under the projection beam in a given reference direction (the xe2x80x9cscanningxe2x80x9d direction) while synchronously scanning the substrate table parallel or anti-parallel to this direction; since, in general, the projection system will have a magnification factor M (generally less than 1), the speed V at which the substrate table is scanned will be a factor M times that at which the mask table is scanned. More information with regard to lithographic devices as here described can be gleaned, for example, from U.S. Pat. No. 6,046,792, incorporated herein by reference.
A projection apparatus, such as used in lithography, generally includes an illumination system, referred to hereafter simply as an illuminator. The illuminator receives radiation from a source, such as a laser, and produces an illumination beam for illuminating an object, such as the patterning means (e.g. a mask on a mask table). Within a typical illuminator, the beam is shaped and controlled such that at a pupil plane the beam has a desired spatial intensity distribution. This spatial intensity distribution at the pupil plane effectively acts as a virtual radiation source for producing the illumination beam. Following the pupil plane, the radiation is substantially focussed by a lens group referred to hereafter as xe2x80x9ccoupling lensxe2x80x9d. Said coupling lens couples the substantially focussed light into an integrator, such as a quartz rod. The function of said integrator is to improve the homogeneity of both the spatial and angular intensity distribution of the illumination beam. The spatial intensity distribution at the pupil plane is converted to an angular intensity distribution at the object being illuminated by said coupling optics, because the pupil plane substantially coincides with the front focal plane of the coupling optics. Controlling the spatial intensity distribution at the pupil plane can be done to improve the processing parameters when an image of the illuminated object is projected onto a substrate.
A known illuminator comprises an optical system referred to hereafter as xe2x80x9czoom-axiconxe2x80x9d. The zoom-axicon is a means for adjusting the intensity distribution at the pupil plane. Radiation from the source passes through a first optical component, which generates an angular intensity distribution. Next, the radiation beam traverses a zoom lens. In the back focal plane of the zoom lens a spatial intensity distribution occurs that generally is suitable to serve as a secondary light source in the pupil plane. Hence, the back focal plane of the zoom lens typically substantially coincides with the pupil plane (i.e., the front focal plane of the coupling optics). The outer radial extent of the spatial intensity distribution at the pupil plane can be changed by changing the focal length of the zoom lens. However, the zoom lens must have two degrees of freedom, one to change the focal length of the zoom lens and a second to change the position of the principal planes such that when the focal length changes, the back focal plane remains located at the pupil plane of the illuminator. Due to this functionality, the zoom lens typically consists of several (e.g. at least three) separate lenses in series, several of which are movable. As mentioned above, by tuning the focal length of the zoom lens, the radial extent of the disc-shaped, preferably homogeneous, intensity distribution at the pupil plane can be set. In the following, any preselected, preferred spatial intensity distribution at the pupil plane may be referred to as an xe2x80x9cillumination settingxe2x80x9d.
An axicon, which is located near the pupil plane, generally consists of two elements having complimentary conical shaped faces. Said axicon is used to generate annular spatial intensity distributions, or other spatial intensity distributions with substantially no intensity around their centre, i.e. no on-axis illumination. By tuning the distance between the two conical faces of the axicon, the annularity can be adjusted. When the axicon is closed, i.e. the gap between the conical faces is zero, conventional (i.e. said disc-like) illumination settings can be produced. With a gap between the conical faces, an annular intensity distribution results, with the inner radial extent of the annulus determined by the distance between the two conical faces; on the other hand the zoom lens determines the outer radial extent and thus the width of the annulus. Preselected inner- and outer radial extents of the intensity distribution are often referred to as "sgr"-settings, in particular the "sgr"-inner setting and the "sgr"-outer setting, respectively. Here, "sgr"-inner and "sgr"-outer are a measure for the ratio of the radius in question to the maximum radius of the pupil.
The term xe2x80x9czoom-axiconxe2x80x9d as employed here should be interpreted as referring to a module comprising said zoom lens and said axicon.
Multipole illumination settings can be generated by various means in the known illuminator, for example by modifying said first optical element in front of the zoom lens, such as to appropriately shape the angular intensity distribution, or by inserting aperture plates or blades into the beam path, for instance near the pupil plane, and so on. Further information on a known zoom-axicon module and multipole mode generation are given in U.S. Ser. No. 09/287,014, filed Apr. 6, 1999, incorporated herein by reference.
In the known illuminator, described above, it is apparent that to produce the desired range of illumination settings the zoom-axicon module will generally have several (e.g. five or more) optical components, which can make it expensive to produce, particularly given the fact that several of the elements must be independently movable. A further problem is that the lenses comprising the zoom lens and the two conical elements of the axicon represent a considerable thickness of lens material and a large number of surface interfaces. This means that the transmission efficiency can be poor due to absorption, inefficient coatings, degradation effects and contamination. This problem is exacerbated by the demand for imaging ever smaller features at higher densities, which requires the use of radiation with shorter wavelengths, such as 193, 157, 126 nm. The efficiency of suitable transmissive materials, such as CaF2 and quartz, generally decreases at shorter wavelengths due to increased absorption. The effectiveness of the optical coatings of the components also typically decreases at shorter wavelengths and degradation effects generally become worse. Thus, overall, a significant throughput reduction can occur, due to decreased transmission. Another problem is that the known illuminator occupies a relatively large volume in the lithography apparatus. This in turn can lead to excess bulk in the machine, and increased manufacturing costs (particularly when using material such as CaF2).
An object of the present invention is to provide an improved lithography apparatus with an illuminator which avoids or alleviates the above problems.
According to one aspect of the present invention, there is provided a lithography apparatus as specified in the opening paragraph, characterized in that said adjusting means consist of at least one exchanger for inserting and removing at least one of a plurality of optical elements into and out of the projection beam path, each of the said optical elements defining at least one parameter of said intensity distribution.
The term xe2x80x9coptical elementxe2x80x9d as employed here should be interpreted as referring to elements such as a diffractive optical element (e.g. comprising an array of diffractive microlenses), referred to hereafter as a xe2x80x9cDOExe2x80x9d, a microlens array, a holographic optical element (e.g. comprising an array of computer generated holographic optical sub-elements, etc. Further information on DOEs is given, for example, in U.S. Pat. No. 5,850,300, incorporated herein by reference. Said elements are generally relatively thin and can be made, for example, on a substantially plane-parallel substrate.
The use of an exchanger and a plurality of optical elements enables the illumination setting of the apparatus to be changed, but without requiring a zoom-axicon module. Elimination of the zoom-axicon module and replacement of this module by a single lens greatly reduces the number of optical components in the illuminator, which reduces the number of surfaces and decreases the thickness of transmissive material necessary, and so significantly improves the initial throughput. Elimination of the zoom-axicon module also lowers the sensitivity of the illuminator to degradation and contamination effects, and can enable the costs of the apparatus to be reduced. Also the size of the apparatus can be reduced.
An apparatus according to the above aspects of the invention can also have the following advantage. In conventional illuminators, the relative complexity and placement accuracy of the optical components can cause an undesired ellipticity in the pupil intensity distribution. However, the relatively simple construction of the present illuminator alleviates this problem.
In a particular embodiment of the invention the position along the beam path of said optical element(s) and the position of said pupil generating lens are adjustable. This enables some continuous variation of the intensity distribution parameters, such as the inner and outer radial extent, to be achieved. This can be used to eliminate the discrete nature of the parameters of the intensity distribution provided by the optical elements or to reduce the number of interchangeable optical elements which need to be provided for the exchanger.
According to one embodiment, at any one time, a single optical element is present in the beam path (after initial beam expansion) and defines a set of parameters of the intensity distribution. The exchanger can swap the optical element for one of a number of different optical elements, each for defining a different illumination setting. This arrangement can be advantageous because the smaller the number of optical elements in the beam, the less the beam is attenuated. This is particularly advantageous for microlens arrays and other optical elements such as DOEs, which are optically thin and relatively inexpensive to manufacture, so it is not a problem to provide a library of different exchangeable optical elements, one for each desired illumination setting.
In an alternative scenario, the exchanger is embodied so as to be able to position at least two of the said optical elements in parallel (i.e. side by side) in the beam path. Alternatively, a single such optical element can be embodied in such as way as to contain a plurality of zones, each zone corresponding to a different type of illumination setting, and the apparatus can be embodied so as to be able to direct the beam through one or more different zones of the said element. For example, the exchanger in the latter scenario may be able to enact fine x,y-motion in a plane parallel to that of the optical element, thus allowing the beam to be finely positionable (in an x,y-plane) with respect to the element; alternatively, optical splitters/mixers may be used to direct different portions of the beam through different zones, or different areas of a single zone, or separate elements, at preselected ratios. All of these scenarios have in common the fact that they allow a great(er) plurality of illumination settings to be achieved on the basis of a relatively small number of the said optical elements, by virtue of the efficient xe2x80x9cmixing effectsxe2x80x9d described above. Moreover, one is enabled in this manner to create (variable) illumination settings that would otherwise be difficult, or impossible, to achieve. One can even contemplate varying an illumination setting during an actual exposure.
Specific examples of xe2x80x9chybridxe2x80x9d illumination settings that can be generated in the matter described in the previous paragraph include:
xe2x80x9csoft multipolexe2x80x9d, where a multipole pattern is generated by a first element (or zone, or area of a zone) and a background flux is generated by a second element (or zone, or area of a zone);
a quadrupole pattern comprising a xe2x80x9cstrongxe2x80x9d dipole in the x-direction and a xe2x80x9cweakxe2x80x9d dipole in the y-direction;
a xe2x80x9cstaggered quadrupolexe2x80x9d pattern, wherein the pole spacing in the x-direction is different to that in the y-direction.
It should be noted that the embodiment described in the previous two paragraphs is not limited in its use to an illuminator in which a zoom-axicon is absent, and replaced by a single lens. In situations where the material costs, absorption issues and/or bulk of the zoom axicon are not a substantial issue (e.g. in DUV lithography, or xe2x80x9chigh-endxe2x80x9d machines for use at 193 nm or 157 nm), this xe2x80x9cmixing embodimentxe2x80x9d might be used in combination with a zoom-axicon, for added flexibility.
In another embodiment of the invention, several of said optical elements are arrangeable in series along the beam path. In one embodiment, a first optical element defines a particular sub-set of parameters of the intensity distribution and a second optical element defines for instance a particular change of said sub-set of parameters. For instance, a first optical element can define a certain preselected annularity defined by an inner and outer radial extent (the "sgr" settings; respectively the "sgr"-inner setting and the "sgr"-outer setting) of the intensity distribution, and a second optical element can have the effect of reducing and increasing (by a preselected amount) said inner and outer radial extent, respectively. This has the advantage that, by placing preselectable combinations of different optical elements in series in the optical path (including the combination of an optical element in one exchanger and an open position in an other exchanger), the number of illumination settings can be increased while minimizing the number of optical elements required. Such an embodiment may comprise several (e.g. two) exchangers in series. The use of, for example, microlens arrays or DOEs as the optical elements is advantageous because generally they are relatively thin and, therefore, even with several of them in series, the degree of light absorption compared with a zoom-axicon module is greatly reduced.
According to a further aspect of the invention there is provided a device manufacturing method comprising the steps of:
providing a substrate that is at least partially covered by a layer of radiation-sensitive material;
providing a projection beam of radiation using an illumination system;
using patterning means to endow the projection beam with a pattern in its cross-section;
projecting the patterned beam of radiation onto a target area of the layer of radiation-sensitive material,
xe2x80x83characterized by:
setting the inner and/or outer radial extent of the intensity distribution of the projection beam using at least one exchanger to position at least one of a plurality of optical elements in the projection beam path, the or each such optical element defining at least one parameter of said intensity distribution.
In a manufacturing process using a lithographic projection apparatus according to the invention a pattern (e.g. in a mask) is imaged onto a substrate that is at least partially covered by a layer of energy-sensitive material (resist). Prior to this imaging step, the substrate may undergo various procedures, such as priming, resist coating and a soft bake. After exposure, the substrate may be subjected to other procedures, such as a post-exposure bake (PEB), development, a hard bake and measurement/inspection of the imaged features. This array of procedures is used as a basis to pattern an individual layer of a device, e.g. an IC. Such a patterned layer may then undergo various processes such as etching, ion-implantation (doping), metallisation, oxidation, chemo-mechanical polishing, etc., all intended to finish off an individual layer. If several layers are required, then the whole procedure, or a variant thereof, will have to be repeated for each new layer. Eventually, an array of devices will be present on the substrate (wafer). These devices are then separated from one another by a technique such as dicing or sawing, whence the individual devices can be mounted on a carrier, connected to pins, etc. Further information regarding such processes can be obtained, for example, from the book xe2x80x9cMicrochip Fabrication: A Practical Guide to Semiconductor Processingxe2x80x9d, Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN 0-07-067250-4, incorporated herein by reference.
Although specific reference may be made in this text to the use of the apparatus according to the invention in the manufacture of ICs, it should be explicitly understood that such an apparatus has many other possible applications. For example, it may be employed in the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, liquid-crystal display panels, thin-film magnetic heads, etc. The skilled artisan will appreciate that, in the context of such alternative applications, any use of the terms xe2x80x9creticlexe2x80x9d, xe2x80x9cwaferxe2x80x9d or xe2x80x9cdiexe2x80x9d in this text should be considered as being replaced by the more general terms xe2x80x9cmaskxe2x80x9d, xe2x80x9csubstratexe2x80x9d and xe2x80x9ctarget areaxe2x80x9d, respectively.
In the present document, the terms illumination radiation and illumination beam are used to encompass all types of electromagnetic radiation, including ultraviolet radiation (e.g. with a wavelength of 365, 248, 193, 157 or 126 nm) and EUV.